What is the best age to start taking men's multi vitamins? he is 24 almost 25.

No vitamins: Vitamins are not needed at any age unless you have a proven deficiency. It's been shown in a recent study that patients taking vitamins "just because" were at a higher risk for dying. Your food should provide you with enough vitamins and if you have any diseases that are causing vitamin deficiency, they need to be identified and treated accordingly.

Shouldn't need it: If you have a reasonable diet, especially if you are only 25 years old, taking vitamin supplements is not really all that helpful. It is much more beneficial to obtain those vitamins via good eating habits, because many of the foods that contain the vitamins have other protective properties (fiber, anti-oxidants, etc.) that will not be obtained by taking a pill.